This invention relates to instrumentation for measurement of physical signals. More particularly the invention can be utilized in a system for the measurement of the amplitude of a physical signal, such as light, sound, vibration or pH (acidity). However, in the broad sense, this invention has application to the measurement of any wide dynamic range signal.
In the measurement of physical signals, such as sound, there must always be a transducer, for example, in the case of sound, a microphone, which converts the physical signal into an electrical signal. The transducer's electrical signal must have a dynamic range as wide or wider than the dynamic range over which the measurement system is intended to accurately function, and, in general, the electrical signal must be processed in some manner in order to extract or compute the particular parameter of interest, for example, the level of sound.
The specifications of modern measurement systems are complex. An example is the International Electrotechnical Commission (IEC) specification for sound level meters. As of this writing, the IEC is preparing a new standard for sound level meters called IEC 1672. The October 1996 draft, entitled Second Committee Draft 1672: Electroacoustics--Sound level meters, is the proposed revision of the long-standing IEC 651, first issued in 1979. This document thoroughly defines the required functionality of sound level meters of various types. Sound pressure is the root-mean-square of the instantaneous sound pressure over a time interval and sound level is the relative frequency-weighted and time-weighted sound pressure, expressed in decibels. The reference sound pressure (0 dB SPL) is 20 microPascals or 0.0002 dynes/cm.sup.2 and both frequency and time weighting are specified in the IEC standard. The sound pressure of an isolated acoustic event of duration T can therefore be evaluated by integrating the square of the instantaneous pressure over the interval F and then computing the square root, provided that T is short compared to the time constant employed for exponential time-weighting.
In this and earlier sound level meter (SLM) standards, various levels of performance are defined and specified. Type 1 meters require extremely stable and accurate microphones and very tight circuit tolerances and are, therefore, expensive. Type 2 meters need not exhibit as high a level of performance. The basic accuracy of a Type 2 meter is .+-.1 dB over a temperature range from -10.degree. C. to +50.degree. C. linearity over the entire range must be .+-.1 dB, with greater linearity (.+-.0.5 dB) for small changes in level. A Type 2 SLM must have a minimum dynamic range of 60 dB, however, users expect it to be wider. In order to measure sound level as defined above, a sound level meter must include certain components and have certain capabilities. FIG. 1 illustrates a basic sound level meter system indicated generally at 10. A transducer, for example, a microphone 15 is connected to an amplifier 20. Audio amplification is necessary to increase the level of the signal from the microphone 15 for the processing of weak sounds, and audio filtering circuitry 25 is necessary to achieve the frequency-weighting requirement. The key element which extracts the sound level from the filtered and amplified microphone signal must be a detector with specific characteristics, either a root-mean-square (RMS) type or a mean-square type. In this case, an RMS type detector 30 is shown. Exponential time-weighting is incorporated using a filter 35 within the RMS detector circuitry. Finally, the exponentially time-weighted RMS level is converted to decibels at block 40 and displayed at a display 45. The output of the RMS detector 30, V.sub.RMS is converted in block 40 to decibels by the relationship dB=20 log(V.sub.RMS /V.sub.ref), where V.sub.ref is the RMS detector's theoretical output at the reference sound pressure level, 0 dB SPL.
As shown in FIG. 1, frequency-weighting takes place prior to detection of the signal. Two types of frequency-weighting are specified for SLMs, "A" and "C", selected by switch 50. A-weighting is generally used for the measurement of low level sounds, because it roughly represents hearing sensitivity at low intensities, and C-weighting is generally used for the measurement of high level sounds.
Time-weighting takes place during and/or after detection of the audio signal. Time-weighting is necessary because observers of physical phenomenon are not generally concerned with activity which takes place over very short durations the duration being dependent upon the nature and purpose of the measurement. For example, transient acoustic events which are so short as to be inaudible or too short to cause any hearing damage will be of no concern to the general user of a sound level meter. Thus, there is always some time-related specification involved in the measurement of physical phenomena. Most acoustic events present a continuously varying level of sound. Sometimes, high intensity transients are present along with a steady background level. A system which continuously measures a physical parameter, such as sound, must, therefore, present some sort of "running average." The standardized method of calculating a running average sound pressure is to utilize exponential time-weighting. In the measurement of sound, two common exponential time-weighting options are generally employed, referred to as "FAST" and "SLOW", selected in FIG. 1 by using switch 55. FAST time-weighting involves a 125 millisecond time constant at filter 35 and SLOW time-weighting involves a 1 second time constant at filter 35.
RMS detection is at the heart of most sound level measurements. Various hardware methods are used to compute RMS, including thermal conversion which has been utilized in what are known as true RMS-reading AC voltmeters. In a thermal conversion type meter, the incoming signal is amplified and actually heats up a resistive element. The temperature rise of that element is measured and indicates the average power or mean square value of the signal. Time-weighting takes place because of the finite thermal mass of the element and also due to the conduction of heat out to the environment. Although simple in concept, thermal conversion is difficult to implement. However, it remains one of the best and most popular methods of RMS detection.
Alternatively, circuitry as shown in FIG. 1 has been constructed to explicitly perform the RMS calculation by first squaring the input signal at a squarer 60, then averaging it at the filter 35, and finally computing the square root at block 65. The design of accurate wide dynamic range explicit analog computational circuits, such as one which derives the square root, are challenging, and, therefore, other methods have been developed. RMS detectors are available which incorporate indirect or implicit computation of the square root, accomplished, for example, by means of feedback and analog division. Implicit detection generally provides wider dynamic range at lower cost when compared to explicit means. The averaging components of both explicit and implicit RMS detectors generally provide exponential time-weighting.
The final step in deriving the sound level is to convert the frequency and time-weighted sound pressure to decibels. This involves the computation of the logarithm, generally accomplished with circuitry that relies on the logarithmic relationship between the collector current in a transistor and its base-emitter voltage. This relationship is well-known to those familiar with semiconductor physics and involves a strong dependency on temperature. It is important, therefore, to provide temperature compensation whenever this analog type of logarithm computation is employed.
A problem encountered in the design of wide dynamic range measurement systems such as sound level meters is that the dynamic range of the detector and/or the circuit which converts to decibels is often smaller than desired, thus limiting the performance of the system. An alternative is to utilize more sophisticated circuitry with wider range, however, doing so increases the system cost. It must be understood than an ideal measurement system would cover its entire dynamic range without any operator intervention. In such a system, when measurements are taking place, the instrument would acquire valid readings over its entire dynamic range even while unattended. When the usable dynamic range of a detector in a measurement system is limited, i.e., smaller than the dynamic range of the instrument itself, it becomes necessary to provide the user with a switch to manually set the range of the instrument. This switch would generally either control the amount of gain or attenuation which precedes the detector such that signals to be measured always fall within the range of the detector. Manually operated range switching is generally undesirable because it implies the need for an extra step on the part of the user to ensure that the instrument is in a range appropriate to the incoming signal.
In some measurement systems, range switching is performed automatically by the instrument itself. In these automatic ranging systems, there is generally a transient in the measurement data which occurs around the time of the range switching. The resulting gap of valid data is sometimes unimportant, as in the case of an autoranging laboratory voltmeter. However, in the measurement of many physical phenomena, gaps or invalid data is undesirable.
As pointed out above, time-weighting is an important aspect of measurement systems. To implement a system with both FAST and SLOW modes, common designs of sound level meters, for example, incorporate the switch 55 as shown in FIG. 1 which modifies the RMS detector's time constant. It is often desirable to be able to control the exponential time-weighting remotely, e.g. by means of a personal computer attached to the instrument through a serial port. It is also desirable to be able to set the time-weighting to any of a variety of values under computer control.
In addition, providing useful measurement data at very low input signal levels is a common problem with known measurement systems. At low levels, microphone (transducer) and amplifier noise in the measurement system interfere with accuracy. A solution to this problem is to use less noisy and more expensive components. However, it is desirable to improve the low-level capability of measurement systems without significantly increasing cost.
Another problem with some wide dynamic range measurement systems is related to operation over temperature variations. Sound level meters, for example, are expected to be accurate at temperatures ranging from -10.degree. C. to +50.degree. C. This is especially challenging for analog computation circuitry. It is desirable for measurement systems such as sound level meters to be accurate over a wide range of temperatures.
An example of high-quality Type 2 sound level meter is the Quest Technologies Model 2400. In this meter, the RMS detector and circuitry for converting to decibels have a dynamic range of 70 dB. The device has two different switch-selectable overlapping ranges: 30 dB to 100 dB and 70 dB to 140 dB. In order to enable the instrument to measure over its entire 110 dB dynamic range, the user must intervene to switch amplitude ranges and ensure that the set amplitude range encompasses the levels being presented to the instrument for measurement. The techniques used in the design of this instrument to provide its 110 dB dynamic range include the use of a high-quality RMS detector, the use of a wide range circuit for computing the logarithm, and the necessary user-operated range switch to select one of two overlapping amplitude ranges.
The Larson-Davis Model 700, on the other hand, has a detector and decibel computation circuit which is accurate over a full 100 dB of dynamic range and is also a Type 2 instrument. This device has no range switch and is a more expensive and sophisticated device because, as the dynamic range is increased, accuracy, linearity, and temperature compensation are all more difficult to attain.